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Spinel is in the aluminium oxide mineral family, and spinel is found primarily within metamorphic rock, and as a primary mineral in basic rock. Spinel is also common within peridotite (olivine, pyroxene), an igneous rock in the boundary between the Earth's crust and the mantle (Mohorovicic discontinuity, or "Moho"). In molten rock or magma, the absence of alkalis (sodium hydroxide or calcium hydroxide) prevents the formation of feldspar. If aluminum oxide is present in the magma, corundum (ruby) will form, or the aluminum oxide will combine with magnesium to form Spinel. The "spinel group" is a collection of minerals which all crystallize in the isometric crystal system, and include the gem spinel, as well as chromite, franklinite, gahnite, hercynite, jacobsite, magnetite, ulvšspinel, ringwoodite, and trevorite.  Ruby and red Spinel derive their coloration from the same Chromium Cr(3+) ion plus trace amounts of iron, manganese, and zinc. This is why spinel and ruby are often found imbedded in the same matrix. The crystal habit of spinel is octahedral or twinned octahedral (macles), and to a lesser extent, cubic or dodecahedral. Some spinels show a "color-change" effect, turning from grayish-blue in daylight or fluorescent light to purple under incandescent light [3]. Spinel Crystallography, Chemistry, Physical Properties
Spinel Optical Properties
Spinel's refractive index and specific gravity can be the same as specimens of pyrope garnet (SG: 3.6, RI: 1.73 - 1.76), can be distinguished by their fluorescence and inclusions. Natural spinel does not fluoresce strongly under short wavelength UV light, but some synthetic spinels do [4]. Spinel Mining in Myanmar & Sri LankaThe highest quality transparent blood-red "ruby spinel" and hot-pink spinel has come from the Pein Pyit (Painpyit) mine, Bawpadan mine, Inn Gaung Pyant mine, and Pingutaung mine in Mogok, Upper Burma (Myanmar) although these sources are rapidly being depleted. Other significant sources are located in the Mong Hsu stone tract, Myanmar and in Sri Lanka. Due to its high weather resistance, spinel is principally found as crystal pebbles in secondary placer deposits (Myanmar: below, left and Sri Lanka: below, right). Although several times more rare than ruby, fine Burmese spinel is much less expensive.
Yellow spinel is called "Rubicelle" and violet colored spinel is called "Almandine." Spinel occurs in cherry-red (Gemmy), orange, hot pink, mauve, blue, dark green (Gahnite), brown, and black. Synthetic SpinelThe spinel series of minerals melt congruently and recrystallize instantly on cooling making it easy to synthesize. Like corundum (ruby and sapphire), spinel is synthesized by the Verneuil, or "flame-fusion" method by melting aluminum oxide and allowing it to re-crystallize. Many synthetic gemstones such as synthetic aquamarine, peridot, ruby, and sapphire are spinel. The color of synthetic spinel is controlled using chemical additives. Modern methods of synthesizing spinel and corundum, such as the Czochralski Pulled-Crystal, Flux-Grown, and Hydrothermal methods, produce more realistic imitations of ruby and sapphire. These synthetics are identified by their characteristic inclusions. Natural spinel, like garnet, is one of the few stones that is not normally enhanced or heat treated in any way. 
Bibliography and Reference on Spinel 1. Field Gemology Ruby, Emerald and Spinel Mining in Central Asia 2. International Colored Gemstone Association Ruby, Sapphire, & Spinel 3. GIA Spinel - History's Most Under-Appreciated Gem 4. Department of Geological Sciences Distinguishing Properties of Spinel 5. Judith Crowe, The Jeweler's Directory of Gemstones 6. Walter Schumann, Gemstones of the World 7. Renee Newman, Gemstone Buying Guide | Copyright © 2012 AllAboutGemstones.com. All rights reserved. | | | ||||||||||||||||||||||||||||||||||||||||||||||||
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: Spinel
